Trocholites contractus Schröder, 1891

Trocholites contractus Schröder, 1891

Figs 1B View Fig , 15 View Fig ; Table 6 View Table 6

Trocholites contractus Schröder, 1891: 19 (157), pl. 1(24) fig. 2.

Trocholites contractus – Sweet 1958: 95, pl. 7, figs 1–2. — Neben & Krueger 1971: pl. 32 figs 6–7. — Dzik 1984: 43, text-figs 11, 12.52, pl. 5 fig. 4, pl. 6 fig. 1.

? Trocholites contractus – Dzik 1984: pl. 5 fig. 4, pl. 6 fig. 1.

Diagnosis

Species of the genus Trocholites with an adult conch diameter of about 33 mm. Conch in the last whorl thickly discoidal (CWI ~ 0.46) and evolute (UWI ~ 0.53) with low coiling rate (WER ~ 1.60). Whorl profile moderately depressed (WWI ~ 1.95) with rounded venter and flanks. Whorl width and whorl height decreasing in last half volution (CWI ~ 0.70–0.50; wh / dm ~ 0.30–0.20). Siphuncle marginodorsal in position.

Type material

Neotype (designated here)

GERMANY – Berlin • Lübars; Lasnamägi Regional Stage ; Neben and Krueger Coll.; figured by Neben and Krueger (1971: pl. 32 figs 6–7), re-illustrated here in Fig. 15 View Fig ; MB.C. 32169 .

Description

Neotype MB.C.32169 ( Fig. 15 View Fig ) is an adult, well-preserved internal mould with a conch diameter of 33 mm; almost 3 whorls are preserved. The last half volution shows conspicuous changes in the shape of the conch morphology, where the terminal body chamber undergoes a significant reduction of width and height. During the last half whorl, in which the conch diameter increases from 26 mm to 33 mm, the general morphology changes from thinly pachyconic to discoidal ( CWI decreases from 0.65 to 0.46) and from subevolute to evolute (umbilicus widens, UWI increases from 0.43 to 0.53); the last half whorl becomes less strongly depressed ( WWI decreases from 2.12 to 1.99), the coiling rate is low ( WER = 1.61) and the last whorl is weakly embracing ( IZR = 0.08) at the mature aperture. The body chamber clearly decreases in width starting from its beginning (from about 17 mm to 15 mm), and also in height starting at about its mid-length (from 8.8 mm to 7.5 mm; wh /dm = 0.30–0.23). The phragmocone chambers are moderately long ( RCL = 0.23).

Remarks

Trocholites contractus was described by Schröder (1891: 19 (157), pl. 1(24) fig. 2; see also Fig. 1B View Fig herein) on the basis of a single specimen from the locality “Rosehnen in Samlande”, present day Priboi on the Samland Peninsula, Kaliningrad Region of Russia. The specimen came from a block of the Echinosphaerites Limestone (Darriwilian, Middle Ordovician) and Schröder (1891) stated that it had been kept in the “Regional Museum of Königsberg” (Kaliningrad). Since that collection was most probably destroyed during World War II (pers. com. D. Weyer, Berlin, 2022), a neotype has been selected and described here. The proposed neotype corresponds stratigraphically to the holotype. Both specimens do not differ significantly from each other morphologically.

Trocholites contractus differs from other species of the genus in the whorl profile; its whorls are wider, more depressed and more strongly embracing. In these aspects, the neotype of T. contractus resembles the preadult growth stage of T. hospes ( Fig. 12 View Fig ). However, individuals of T. hospes attain a much larger adult size (up to 60 mm in diameter), while in T. contractus the size of the conch in the terminal stage is only about 33 mm. Furthermore, a characteristic feature that readily distinguishes T. contractus from T. hospes and any other trocholitid species is the marked and rapid decrease in whorl width, whorl height and impression rate of the terminal body chamber.

Geographic and stratigraphic occurrence

Kaliningrad Region of Russia, northern Germany, Poland, Norway, Estonia; Darriwilian, Middle Ordovician.